1. Field of the Invention
This invention relates to the remote sensing of pressure drops across an orifice plate in a pipeline and particularly to an orifice saddle to facilitate, stabilize and reduce vibration of the transmitter and associated apparatus, such as valved manifold blocks connecting the transmitter to taps straddling the orifice plate.
2. Description of the Related Art
The gas industry makes extensive use of orifice plates for volume measurements. Orifice plates are artificial constrictions in a pipeline. As a result of this constriction there is a pressure drop from the upstream side of the orifice plate to the downstream side of the orifice plate. There is a relationship between the pressure drop and the amount of gas flowing through the pipeline. The pressure drop can be measured which in turn allows a determination of the flow through the pipeline.
The pressure drop is measured using electronic transmitters. These transmitters actually measure the pressures on both the upstream and downstream sides of the orifice plate and then record the difference between the two. Devices of this type are referred as differential pressure transmitters. There are numerous manufacturers of this type of device (e.g. Rosemount, Honeywell, Foxboro and others).
Orifice plate fittings (such as the type manufactured by Daniel Industries) and orifice flanges (manufactured by Daniel and others) are provided with holes (called taps) both upstream and downstream of the orifice plate. The taps are generally threaded holes, typically xc2xdxe2x80x3 female NPT (National Pipe Threadxe2x80x94a tapered thread designed to seal pressure tight).
It is necessary to incorporate valving systems between the orifice taps and the differential pressure transmitters. These valving systems need to include the following functions. Block valves to isolate the flow through the upstream and downstream taps (generally referred to as the high pressure side and the low pressure side), between the orifice flange or fitting and the transmitter. An equalizer valve, or valves, to control the connection between the high pressure passages and the low pressure passages. A vent valve, or valves, to allow trapped pressures to be bled off or to allow the passages to be purged of entrapped gases or liquids. Sometimes the vent valve(s) are incorporated into the transmitters.
The natural gas industry has evolved a number of specifications and requirements to minimize errors in the measurement of flow and to provide improved accuracy and response time. Some of these are:
1. The closer the transmitter is to the orifice plate the better.
2. The orifice size through the valving system from the orifice taps to the transmitter must be xe2x85x9cxe2x80x3 in diameter.
3. The flow passages between the orifice taps to the transmitter should be as straight as possible. The ideal is a straight through passage. One 90 degree turn in the flow passage is permitted.
Another consideration is mounting the valve system and the transmitters to the orifice plate assembly. Conventional mounting, valving, and transmitter assemblies can weigh 40 pounds and, depending on the complexity of the assembly and the products used, up to 70 pounds. The pipeline system and the orifice flanges or fittings are subject to vibration. The valving and transmitter assemblies also tend to be leaned on by the instrumentation personnel. It is necessary to spread the loads arising from these factors off of the typical xc2xdxe2x80x3 NPT tap connections and transfer the loads onto the orifice fitting or flange. Currently available products use individual taps with attachments to spread the load. These are called stabilized connection flanges or stabilized futbol flanges. An example of such a stabilized connector flange is shown in U.S. Pat. No. 4,920,626 assigned to Precision General, Inc. of Houston, Tex.
In this patent is provided a stabilizer foot which is a generally parallelpiped body portion and/or containing pair of stabilizer feet attached to the body portion which feet project axially toward the orifice plate assembly. The body portion is provided with a bore through which is fitted an NPT threaded connector flange so that the body and/or the stabilizer feet are radially outward of the NPT threads and provide outriggers for engagement with the external surface of the orifice plate assembly.
However, if the orifice plate assembly is curved the body and/or stabilizer feet only make point contact along the curved surface. The provision of a flat boss on the orifice plate assembly will increase the contact with the parallelpiped body surface and/or projecting stabilizer feet but then the requirement of a planar boss limits the applicability of the usage of the device in U.S. Pat. No. 4,920,626 to only certain types of orifice fittings.
Notwithstanding the use of the device of U.S. Pat. No. 4,920,626 on the limited planar or curved surfaces, the shear forces exerted by the dead load of the transmitters (and any associated block manifolds) and live load exerted by instrumentation personnel leaning on the transmitter (and block manifold) will be borne by the NPT threads tending to disturb the seal between the threads and the orifice taps, resulting in the possibility of leaks.
Thus, there is a continuing need to provide an alternative connection method and apparatus to operatively fluidly couple the high and lower pressure taps about an orifice plate with a transmitter.
The present invention is directed to a method and apparatus for operatively fluidly connecting the high and low pressure taps of an orifice plate with a transmitter.
In a particularly preferred embodiment an orifice saddle is provided which orifice saddle preferably has one round hole and one elongated hole, though two round holes can be provided.
The threaded end of the taps are inserted through these holes and tightened into the xc2xdxe2x80x3 NPT holes of the orifice fittings or orifice flange. The end of the taps, distal from the threaded end tightened into the xc2xdxe2x80x3 NPT holes, is known as the xe2x80x9chex endxe2x80x9d due to their configuration and are larger than the threaded end.
The saddle is forced outward of the taps toward the hex end by a series of adjustable feet which may take the form of adjustable stabilizing bolts extending from the saddle toward the orifice assembly.
This forcing places the taps in tension and forms a stable platform to attach the rest of the assembly. Any further live or dead loads are carried by the stabilizer bolts.
When the nature of the orifice fitting does not provide a sufficient area upon which the stabilizer bolts may bear, in a further embodiment of the invention a lower plate may be provided to increase the area of the orifice fitting assembly upon which the adjustable stabilizing bolts may bear.